| Year | Value | | 2020 | 1.7476406850752 | | 2019 | 1.7416292942047 | | 2018 | 1.7604834575452 | | 2017 | 1.7788015769076 | | 2016 | 1.7764112693743 | | 2015 | 1.724756151252 | | 2014 | 1.7527796548408 | | 2013 | 1.7617734567966 | | 2012 | 1.9254642649033 | | 2011 | 1.9708853554142 | | 2010 | 2.0184555893293 | | 2009 | 2.0702270031149 | | 2008 | 2.0980928336142 | | 2007 | 2.1565233393333 | | 2006 | 2.2178620696793 | | 2005 | 2.2817540299579 | | 2004 | 2.3447428520514 | | 2003 | 2.4057044062882 | | 2002 | 2.4694488025643 | | 2001 | 2.5386552573858 | | 2000 | 1.5854625282029 | | 1999 | 1.6316314162561 | | 1998 | 1.6801819766326 | | 1997 | 1.1223728865148 | | 1996 | 1.5872370654697 | | 1995 | 1.5984591643019 | | 1994 | 1.611850323579 | | 1993 | 1.6613503455609 | | 1992 | 1.7141081816526 | | 1991 | 1.7688001344288 |
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